The purpose of this study is to determine the mechanism by which the lung accumulates large amounts of 1,2 dipalmitoyl-sn-glycerol-3-phosphorylcholine (DPPC), a major component of pulmonary surfactant. In vitro experiments will be performed on type II alveolar cells to determine which reactions are important in DPPC synthesis and to determine the mechanism for the accumulation of DPPC in lamellar bodies. Similar studies will be performed on A549 cells, a model system for studying surfactant synthesis. A second objective is to determine the step in the CDP-choline pathway that accounts for increased synthesis of phosphatidylcholine, particularly DPPC, in developing lungs. The rates of various reactions in the CDP-choline pathway as well as the retailoring reactions will be determined in primary cultures of mixed lung cells and cultures enriched with type II cells both of which will be prepared from the lungs of rats at various stages of prenatal development.